The operation of the blast furnace in the production of iron involves processes of chemical reduction in which oxides of iron and other metals are reduced and oxygen removed. The furnace is charged with four basic ingredients; (1) iron oxides, in the form of raw ore, beneficiated pellets, briquettes, nodules, sinters, or other agglomerates. (2) calcium carbonate, (I use the term calcium carbonate to include either limestone or dolomite). (3) a fuel usually in the form of coke. (4) air which provides oxygen to support the combustion. The raw iron ore as it comes from the Lake Superior region contains approximately 50% iron in the form of iron oxide Fe.sub.2 O.sub.3 and manganese oxide (MnO), with the remainder being silica (SiO.sub.2), aluminum (Al.sub.2 O.sub.3), magnesia (MgO), lime (CaO), sulfur (S), and phosphorus (P). The sulfur and phosphorus are commonly considered impurities.
The iron oxides, or other metallic charge materials, coke and calcium carbonate are charged into the blast furnace, one at a time, in measured amounts to form layers of iron ore, limestone or dolomite, and coke; and air (wind) is passed through these layers and the coke burned. Burning of the coke produces heat and carbon monoxide which has a part in the chemical reduction of the iron oxides. As the coke burns the iron oxides are reduced and come into the form of molten iron. The limestone, or dolomite, along with quantities of impurities such as sulfur and phosphorus, form a slag. The hearth, which is at a lower part of the furnace, is the hottest part of the furnace and the layers of ore, coke and calcium carbonate keep moving downwardly within the furnace to the hearth. At some point in this movement downwardly in the furnace slag is formed, and after its full passage downwardly in the furnace it is withdrawn from the furnace in the form of liquid slag. The slag is important to the operation of the furnace because it carries with it many unwanted impurities and so separates these from the iron and removes them from the furnace.
When the downward movement of the iron bearing charge materials, the coke and the calcium carbonate proceeds in a uniform way with the movement taking place constantly and evenly on all sides of the furnace, this is evidence of good operation. Unfortunately, this is not always the case.
As is well known to blast furnace operators there are times when the downward movement of the ingredients charged into the furnace is not regular and uniform or when the movement on one side of the furnace is greater than on the other side making the furnace unbalanced or "lopsided". There are even times when the downward movement becomes severely restricted, and then after operation for a time under such conditions the whole mass may let loose descending at once into the hot part of the furnace with the result that the hearth temperature is reduced to below an operable temperature, sometimes almost extinguishing the fire. When this happens the furnace may have to be shut down, cleaned and then restarted, which is a time-consuming and expensive operation.
It is our belief that the faulty operation above referred to is due in large part to the presence in the charge materials of a greater than usual content of alkali metal oxides such as Na.sub.2 O, K.sub.2 O, Li.sub.2 O. These oxides appear to pass downwardly to hotter parts of the furnace and there become volatilized after which they pass up the furnace inwalls with the wind and then condense above the mantle of the furnace forming stable alkali-alumino-silicates. Such alkali-alumino-silicates are believed to lead to a scaffolding effect which prevents the layered column of burden material from descending in a regular uniform manner. A continuation of this action develops a situation where the mass will collapse of its own weight, chilling the furnace hearth where the most important smelting reactions take place.